Electrolytes are usually composed of an ionic salt which is soluble in a coordinating or solvating solvent such as water or in the non-aqueous systems, such as acetonitrile, ethylene carbonate, dimethyl carbonate, sulfur dioxide, etc. The limitations on electrolytes are the solubility of the salt, the nature of the application, i.e., lithium batteries require lithium salts, and non-aqueous capacitors require a quaternary ammonium salt usually tetrafluoroborate. The overall conductivity of the system is important with higher conductivity more important than other factors. The non-aqueous electrolytes are usually from 3 mS/cm up as high as 70 mS/cm, while aqueous systems range up to 140 mS/cm. Another group of electrolytes are the ionic liquids which are conductive salts which have melting points below room temperature such as 1,3-ethylmethylimidazolium tetrafluoroborate (m.p. 15° C.) with a conductivity at 25° C. of 12 mS/cm which is among the highest of the usual ionic liquids. There are many ionic liquids which can have various organic cations or different anions. In capacitors that use electrolytes, the temperature has a major influence on the electrolyte in the performance of the capacitor since the conductivity of the electrolyte decreases with temperature.
There is a continuing need to have liquid electrolytes which function at very low temperatures. <−50° C. and even down to −75° C. with good conductivity, >2 mS/cm. (Aqueous electrolytes are all frozen by −20 or −30° C.). The organic electrolytes in advanced capacitors are generally good down to −30 or −40° C. before they freeze or go to glass. The lowest temperature electrolytes in lithium batteries (primary or secondary) have some conductivity down to −40 to −50° C. before performance stops due to conductivity being reduced to less than 1 mS/cm or they freeze or go to a glass. There are other electronic components such as sensors and specialized displays which are dependent on liquid electrolytes which also will limit or stop performance below −50° C. For electronic applications at high altitude, in space, in satellites or outside of planes in flight, where temperatures below −50° C. are encountered routinely, there is a need for such low temperature electrolytes. Currently these systems must have additional heat input and are insulated to function, all contributing to extra power usage and extra weight.
U.S. patent application Ser. No. 11/647,713 to Smith et al, which is herein incorporated by reference, relates to electrolyte solutions with tetraalkylammonium tetrafluoroborates, N,N-dialkylpyrrolidinyl tetrafluoroborates, and spiro-pyrrolidinyl tetrafluoroborates in a eutectic mixture of non-nitrile solvents with ethylene carbonate for use over a wide range of temperatures. The electrolytes freeze at about 48° C.
U.S. Pat. No. 6,902,683 to Smith et al, which is herein incorporated by reference relates to electrolytes of a complex salt formed by mixing of a tetraalkyl ammonium salt of hydrogen fluoride with an imidazolium compound in a nitrite solvent which operate at temperatures between −60 and 150° C. However, the conductivity decreases at the extremely low temperatures.
The article of Ue in J. electrochem. Soc. Vol 141, No. 11, November 1994 entitled “Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double-Layer Capacitors” which is herein incorporated by reference, discloses high permittivity solvents and onium salts for double-layer capacitors. Specifically studied were quaternary onium tetrafluoroborate salts which showed greater solubility in the solvents.
U.S. Pat. No. 5,418,682 to Warren et al, which is herein incorporated by reference discloses a method of preparing tetraalkyl ammonium tetrafluoroborate salts for use as electrolytes with dinitrile mixtures as solvents.
U.S. Pat. No. 5,965,054 to McEwen et al, which is herein incorporated by reference discloses non-aqueous electrolytes for electrical storage devices utilizing salts consisting of alkyl substituted, cyclic delocalized aromatic cations and their perfluoro derivatives with alkyl carbonate solvents.
U.S. Pat. Nos. 6,535,373 and 6,902,684 to Smith et al, which are herein incorporated by reference, disclose similar electrolytes which utilize nitrite solvents primarily.